1995 SHO MTX stumble at start

[m&dsho]

Hi all,

I have recently become the new owner of a 1995 MTX SHO. I'm in the process of trouble-shooting a starting issue the car has had since I bought it.

After sitting for a couple of hours, the engine seems to crank for an extended period and stumble before it catches and finally starts, but it will always start. If I go for a drive and turn the car off and turn it back on immediately or within a short amount of time, it starts up perfectly. Here is the video of the first start:

My initial search pointed me to a bad IAC. After replacing the IAC, it now starts up quicker than before, but still stumbles before the engine catches. RPMs hover around 200 while cranking, and as the engine stumbles, RPMs increase to just below 500 before the engine finally catches. The check engine light is not illuminated while the engine is cranking. I did the KOEO test and got code 111, which I found means there are no problems.

I have also replaced the battery, fuel filter, spark plugs, air filter, thermostat, coolant, and ran a bottle of injector cleaner as part of maintenance. Wires are fairly new as well. I was planning on replacing the CID because the symptoms sound similar to what others have described, but I wanted to come here and get a second opinion on whether or not that is the problem.

Thanks

 

[zoomlater]

Try unplugging the CID and try and start the car. It may take a few tries. How does it run after that

 

[m&dsho]

Try unplugging the CID and try and start the car. It may take a few tries. How does it run after that

Okay, I will try that. Is that testing for the CID operation? The car runs fine once it starts up, it's just that hiccup when it's starting.

 

[zoomlater]

Rubydist describes it the best:

"When you turn the key to start, with a total lack of cam sensor input, the pcm guesses which cylinder is number one and fires the coils according to that guess. This guess is locked in as long as the key is "start" or "on". The pcm will guess again the next time that the key is turned to start. Therefore, the trick to making this work is SHORT cranking attempts, only long enough to get the engine to start if the spark is going to the right place at the right time."

 

[NoSlo]

The camshaft position sensor would have very distinct symptoms, including the tach acting erratically and the car only starting on half the attempts no matter how long you crank it.

Is it a stumble when fully cold, or only when recently driven? I'm thinking it's a fuel problem, such as vapor lock or the regulator not holding pressure. When hooking a fuel pressure gauge and turning the key, you should hear the fuel pump run for three seconds, and the pressure should hold at 40psi for several minutes after the pump stops and you turn off the key. The diaphragm of the fuel regulator could leak, causing fuel in the intake and bad pressure regulation.

Another component that is overlooked because it rarely needs service is the evaporative emission charcoal canister. It holds fuel tank vapors, and a solenoid opens it to engine vacuum when the car is running to burn the fumes. If the canister is completely saturated with fuel because the solenoid never opens, or dumps vapors into the engine at startup because the solenoid never closes, you can get such symptoms.

Free maintenance is to spray down the MAF sensor with electrical contact cleaner, and then do an "idle reset" - disconnect the battery for 10 minutes to erase keep-alive memory, and when you start the car without touching the gas pedal, let it idle for a few minutes, turning on A/C and all accessories so it can learn to control idle. The voltage of the throttle position sensor can be checked, at idle position it can be tweaked on the bolts so it puts out 0.95v.

 

[rubydist]

does it act flooded when this happens? because my first thought is a leaking fuel injector causing flooding during hot restarts. that would happen for any restart between possibly 10 min to 2 hours after shut off.

if you changed the iac without disconnecting the battery, it can take up to 2 weeks for the pcm to learn the new curve for the new iac, so no conclusions can be drawn until the pcm learns the new iac.

 

[m&dsho]

The camshaft position sensor would have very distinct symptoms, including the tach acting erratically and the car only starting on half the attempts no matter how long you crank it.

Is it a stumble when fully cold, or only when recently driven? I'm thinking it's a fuel problem, such as vapor lock or the regulator not holding pressure. When hooking a fuel pressure gauge and turning the key, you should hear the fuel pump run for three seconds, and the pressure should hold at 40psi for several minutes after the pump stops and you turn off the key. The diaphragm of the fuel regulator could leak, causing fuel in the intake and bad pressure regulation.

Another component that is overlooked because it rarely needs service is the evaporative emission charcoal canister. It holds fuel tank vapors, and a solenoid opens it to engine vacuum when the car is running to burn the fumes. If the canister is completely saturated with fuel because the solenoid never opens, or dumps vapors into the engine at startup because the solenoid never closes, you can get such symptoms.

Free maintenance is to spray down the MAF sensor with electrical contact cleaner, and then do an "idle reset" - disconnect the battery for 10 minutes to erase keep-alive memory, and when you start the car without touching the gas pedal, let it idle for a few minutes, turning on A/C and all accessories so it can learn to control idle. The voltage of the throttle position sensor can be checked, at idle position it can be tweaked on the bolts so it puts out 0.95v.

The stumble only happens during startup, it's perfectly normal after that. I hear the fuel pump running after turning the key, but I don't have a fuel pressure gauge handy. I'll try to get one and test the pressure. Is there a way to test the evaporative emission charcoal canister for a problem, or is it just a replacement item?

Also, I'll clean the MAF too. I had planned on doing it with the other maintenance, but the different screws holding it to the lid of the airbox kept me from getting it out.

Thanks

 

[m&dsho]

does it act flooded when this happens? because my first thought is a leaking fuel injector causing flooding during hot restarts. that would happen for any restart between possibly 10 min to 2 hours after shut off.

if you changed the iac without disconnecting the battery, it can take up to 2 weeks for the pcm to learn the new curve for the new iac, so no conclusions can be drawn until the pcm learns the new iac.

When hot, it restarts normally and does not stumble as it did in the video I posted. It's only after sitting for a while that it does that again. I don't distinctly remember disconnecting the battery after replacing the IAC, but I'll do that along with the idle relearn mentioned in a previous comment and see if that helps with the problem.

Thanks

 

[m&dsho]

Rubydist describes it the best:

"When you turn the key to start, with a total lack of cam sensor input, the pcm guesses which cylinder is number one and fires the coils according to that guess. This guess is locked in as long as the key is "start" or "on". The pcm will guess again the next time that the key is turned to start. Therefore, the trick to making this work is SHORT cranking attempts, only long enough to get the engine to start if the spark is going to the right place at the right time."

I will definitely try this then, but it has been rainy in CA for the past couple of weeks so I have not been able to work on it as much.

Thanks

 

[NoSlo]

The stumble only happens during startup, it's perfectly normal after that. I hear the fuel pump running after turning the key, but I don't have a fuel pressure gauge handy. I'll try to get one and test the pressure. Is there a way to test the evaporative emission charcoal canister for a problem, or is it just a replacement item?

Also, I'll clean the MAF too. I had planned on doing it with the other maintenance, but the different screws holding it to the lid of the airbox kept me from getting it out.

Thanks

You would start with the canister purge solenoid valve. The small hose connected to the top of the throttle body near the IAC leads to the evaporative emissions purge solenoid. It just kind of hangs there in the middle of the hose somewhere below the coil pack.

Textbook diagnosis is done with a hand-held vacuum pump:
- disconnect the hose from the intake, and disconnect the hose from the opposite side of the purge solenoid that leads to the canister inside the fender (don't lose the hose down there, maybe tape it to something...),
- Pump 16 inch vacuum on the hose that was connected to the intake. It should hold the vacuum.
- Disconnect the solenoid harness connector and hook the solenoid up to 12v. It should click, and the vacuum should be released, air should flow.

The easier diagnosis is to disconnect the hose from the intake and cap the intake port (and plug and secure the hose for safety). See if the problem goes away.

If it's stuck open or leaking, your engine could be "flooded" from a dose of fuel tank vapors that have collected in the canister after the car sits for a while.

 

[rubydist]

Is there a way to test the evaporative emission charcoal canister for a problem, or is it just a replacement item?

Typically, the only issue with the charcoal canister is that it can get saturated if the purge valve never opens. There was an issue with some of the PCMs where the driver for the purge valve would fail and therefore the canister would never get purged and get saturated. The symptom of this is a gas smell at the front of the car nearly always and worse with a newly full tank of gas. The solution is typically to just "jump" the purge valve so the canister always purges. I did this on my red 94 and ran that way for years with no issues - always passed emissions fine and no negative driving / performance issues at all. However, I don't think that the charcoal canister has anything to do with your issues.

 

[Dannyboy puckett]

Typically, the only issue with the charcoal canister is that it can get saturated if the purge valve never opens. There was an issue with some of the PCMs where the driver for the purge valve would fail and therefore the canister would never get purged and get saturated. The symptom of this is a gas smell at the front of the car nearly always and worse with a newly full tank of gas. The solution is typically to just "jump" the purge valve so the canister always purges. I did this on my red 94 and ran that way for years with no issues - always passed emissions fine and no negative driving / performance issues at all. However, I don't think that the charcoal canister has anything to do with your issues.

I know this is an old post.sorry to revive this,but I'm experiencing the fuel smell under hood all the time after parking it. I don't see any leaks from lines or regulator area. My question is how do you "jump" the purge valve to always have it purging. Thank you

 

[rubydist]

Its been a long time since I did that, but I think I just removed the purge valve so the intake vacuum is always purging the canister.

 

[Dannyboy puckett]

Its been a long time since I did that, but I think I just removed the purge valve so the intake vacuum is always purging the canister.

So basically you physically remove the purge valve,and disconnect the electrical connector? And that is it? Im confused on this, because I don't know the layout on this system.i have no diagram or picture. Thank you

 

[rubydist]

Correct.